Symmetrical block copolymers of the type ABA or CABAC, wherein B represents a predominantly poly(conjugated diene)block, A represents predominantly poly(monovinylaromatic) blocks, and C represents predominantly poly(polar vinyl monomer)blocks, have shown a growing commercial interest, while on the other hand the requirements to be met by such block copolymers with reference to their specified standard properties have become more and more stringent.
The preparation of dilithium organo compounds for the anionic polymerization of monovinylaromatic and/or conjugated diene monomers into block copolymers was known in principle from e.g. U.S. Pat. Nos. 3,652,516; 3,734,973; 3,663,634; 3,694,516; 3,668,263; 3,903,168; 3,954,894; 4,039,593; 4,172,100; 4,182,818; 4,960,842; European patent applications Nos. 0316857 and 4132294 and from Macromolecules 5, 453-8 (1969); R. P. Foss et al, Macromolecules 12, 344-6 (1979); C. W. Kamienski et al, Curr. Appl. Sci. Med. Technol. 315-25 (1985); R. P. Foss et al, Macromolecules 10, 287-291 (1977); R. P. Foss et al, Macromolecules 12, 1210-1216 (1979); Polymer 23, 1953-9 (1982); T. E. Long et al, J. Polym. Sci. Part A, Polym. Chem. vol. 27, 4001-4012 (1989).
In particular from the U.S. Pat. No. 3,663,634 the preparation of hydrocarbon soluble organodilithium polymerization initiators was known. Said preparation comprised intimately contacting lithium metal with at least one compound selected from the group consisting of polyaryl substituted ethylenes, hydrocarbon substituted and unsubstituted conjugated diolefins and vinyl substituted aromatic compounds containing only carbon and hydrogen atoms, in a solvent mixture comprising:
(A) at least one solvent member selected from the group consisting of aliphatic, cycloaliphatic and aromatic hydrocarbons, and PA1 (B) at least one solvent member selected from a group of aromatic ethers, aromatic thioethers and tertiary amines, and wherein the volume fraction of solvent component (A) in the mixture can range from 57.0 vol % to 92.0 vol % and PA1 wherein the volume fraction of solvent component (B) in the solvent mixture can range from 8.0 to 43.0 vol %.
As component (B) preferably anisole was proposed and as the polyaryl-substituted ethylene 1,1-diphenylethylene was proposed.
The preferred amount of anisole in the solvent mixture was indicated to be about 15 vol % in combination with 85 vol % benzene.
From U.S. Pat. No. 3,694,516 was known a method for the preparation of a solvated lithium metal adduct of at least one member selected from the group consisting of (a) conjugated polyene hydrocarbon monomers and in particular 1,3-butadiene or isoprene, (b) vinyl substituted aromatic hydrocarbon monomers and in particular styrene, (c) mixtures of (a) and (b), and (d) their slightly chain-extended oligomers, which comprised providing a solution, in an organic solvent which includes at least one member selected from volatile liquid inert strongly solvating dialkyl ethers, cyclic ethers and tertiary amines, of at least one di- or poly-lithio adduct selected from the aforesaid (a), (b), (c) and (d) groups, admixing said solution with at least one member selected from the group consisting of weakly solvating liquid ethers (e.g. anisole) and weakly solvating liquid tertiary amines, said weakly solvating compounds having a boiling point substantially higher than the boiling point of said strongly solvating compound, and evaporating from said mixture substantially all of said strongly solvating compound without substantial evaporation of said weakly solvating compound.
The preferred liquid hydrocarbon solvent was benzene.
In U.S. Pat. No. 4,196,154 multifunctional lithium containing initiators were lateron disclosed, which were soluble in hydrocarbon solvents and thus highly useful in the preparation of block copolymers aimed at.
Additional teachings regarding the use of multifunctional lithium based initiators could be found in e.g. U.S. Pat. Nos. 4,201,729; 4,200,718; 4,205,016; 4,172,190 and 4,196,153. However, the disadvantage connected with the use of multifunctional lithium containing initiators, providing polymers having rather wide molecular weight distributions (about 1.1 or greater), still remained.
Additional techniques were proposed, such as the use of coinitiators such as lithium alkoxides or specific triamines such as pentamethyldiethylene triamine or combinations thereof were proposed to obtain butadiene containing block copolymers having molecular weight distributions down to 1.06. Similar polymers containing primarily isoprene did not become available at all, because isoprene appeared to be less easily polymerizable by anionic techniques compared to butadiene and even rapid polymer addition did not occur.
It has been generally appreciated by persons skilled in the art, that block copolymers of improved tensile strength are obtained if the block copolymer has a reduced molecular weight distribution. In particular, a triblock polymer of the formula styrene/isoprene/styrene prepared by conventional coupling of monofunctional lithium initiated diblock copolymers was found to demonstrate significantly improved tensile strength for equivalent number average molecular weight polymers, if the molecular weight distribution (Mw/Mn) is 1.03 instead of 1.20, L. C. Case, Makromol. Chem, V. 37, p 243 (1960).
Therefore there is still a need for block copolymers of monovinylaromatic monomer and conjugated diene, having the presently required physical properties and hence a related appropriate molecular weight distribution.
According to the disclosure of the European patent application No. 0316.857 it was tried to produce the block copolymers aimed at by means of a process, using a very specific diinitiator organo- lithium compound and a specific organic diamine or triamine. Especially against the use of such amines on an industrial scale, objections arose with reference to environmental and health safety aspects.
From the European patent: application No. 0413.294 it was known to produce narrow molecular weight distribution block copolymers (Mw/Mn in the range from 1.027 to 1.058) of the formula: EQU B-B'-X-(B'-B); or EQU A-B-B'-X-(-B'BA)
wherein A was a block of a non elastomeric monomer, B is a block of isoprene, B' is a block of butadiene and X is the remnant of a hydrocarbon soluble difunctional lithium initiator, said block copolymer having a molecular weight distribution (Mw/Mn) of less than 1.06.
As difunctional lithium based initiator were specified 1,3- or 1,4-phenylene bis(3-methyl-1-phenylpentylidene) bislithium or 1,3- or 1,4-phenylene bis(3-methyl-1-(4-methyl)phenylpentyliydene) bislithium.
Moreover the polymerization was conducted in the presence of an aliphatic triamine.
Although the use of dilithio initiators for the preparation of symmetrical block copolymers having an appropriate narrow molecular weight distribution had been disclosed in principle for a long time, up to now no actual commercial polymerization process has been carried out with the use of said initiators, due to an inevitably occurring too high vinyl content in the poly(conjugated diene) blocks caused by the copresence of a polar compound such as amines or ethers.
It will be appreciated that symmetrical block copolymers aimed at, can in principle be manufactured by coupling of living initially prepared intermediate block copolymers with a multifunctional and in particular difunctional coupling agent. However a disadvantage of such coupling process was formed by the presence in the final block copolymer of a usually difficultly controllable amount of diblock copolymer, formed from the intermediate living polymer.
Another alternative preparation route of such symmetrical block copolymers comprises the fully sequential polymerization by using a monofunctional organolithium initiator optionally in combination with a second initiation to provide a predetermined controlled amount of diblock copolymer. However, a problem of said full sequential polymerization process is caused by a relatively broad molecular weight distribution of the block segment, due to a relatively difficult initiation of the last monomer charge.
It will be appreciated that an industrial process for the preparation of symmetrical block copolymers, and in particular symmetrical triblock copolymers, which show an acceptable molecular weight distribution in combination with a relatively low vinyl content in the poly(conjugated diene) blocks due to 1,2 or 3,4 polymerization, which could meet the requirements of modern end uses of said block copolymers, by the use of a difunctional organolithium initiator, may provide significant advantages such as shorter polymerization times.
An object of the present invention was therefore to provide a suitable difunctional organolithium initiator. Additional objects of the present invention was to provide an attractive industrial process for the preparation of symmetrical block copolymers as specified hereinbefore and to provide a process for the preparation of such initiators.